1. Field of the Invention
The present invention relates to an image read apparatus which reads images by an image sensor, and more particularly to an image read apparatus simultaneously performing shading correction and gamma correction of images read out by the image sensor.
2. Description of the Prior Art
The conventional image read apparatus processes image signals as shown in FIG. 1. Namely, one-dimensional image sensor 1 consisting of photoelectric conversion element outputs voltage proportional to density of an original, the voltage being amplified by an amplifier 2 and converted into a digital value by an AD converter 3. Next, the digitized image signal is corrected by a shading correction circuit 4 to correct for nonuniformity in quantity of light from a light source with respect to each element and variation in sensitivity thereof at the image sensor 1, and then by a gamma correction circuit 5 to have the input-output relation matched with the characteristic of an output device. The image signal thus corrected, next, is binary-digitized by a binary digitizing converter 6 for a binary output device, wherein a dither method or a density pattern method is used for representing the halftone.
The shading correction, as disclosed in, for example, the Japanese Patent Laid-Open Gazette No. Sho-55-112685, reads correction data prestored in a correction memory and performs the correction.
The gamma correction has been disclosed in, for example, the Japanese Patent Laid-Open gazette No. Sho-59-161970, Patent Laid-Open Gazette No. Sho-60-91762 and Patent Laid-Open Gazette No. Sho-60-139080.
It is indispensable for the image read apparatus to keep linearity of an input and an output. However, the output images are different from each other in characteristic and picture quality due to the output device, so that it is required for the image read apparatus to process the image signal to match the output device, the processing being carried out by the gamma correction circuit.
The image read apparatus may be connected to a plurality of output devices. In this case, it is disadvantageous from the point of manufacturing when the processing method (circuit) is different for every output device. Hence, the processing should be performed in the same circuit structure.
Also, the method wherein the shading circuit and gamma circuit are separate from each other produces larger quantization error when CCD output variation is larger in comparison to when it is smaller, thereby reducing an amount of the information available for faithfully reproducing the halftone. Also, even when the binary digitizing converter circuit changes its threshold value by seeking a histogram of amplitude of the image signal, the amount of information largely lacking which can possibility lead to a deterioration of picture quality.